A magnetic recording medium, particularly an audio-cassette recording tape has recently been used for recording music, and has been required, therefore, to have better frequency characteristics and better reproducing ability of original sounds. Further, products such as a car stereo and a radio with an audio cassette recorder are becoming more popular which causes an increase in demand for a tape having good running properties and durability.
In a video-cassette tape recording system, the wavelength to be recorded is made shorter, and the track width is made narrower to achieve high density recording. Therefore, it is necessary for a tape to have high output level and reproducing ability of an original image with high S/N ratio. It is also necessary for a recording tape for a video tape recorder (VTR) with the increase of commercial use of portable VTR that a total thickness of video tapes becomes 20 .mu.m or thinner and running properties and durability of the tapes have to be far better than conventional video tapes. That is, audio tapes and video tapes having more excellent electromagnetic properties, running properties and durability are strongly desired.
Various binder compositions have been proposed to improve the above properties in, for example, U.S. Pat. Nos. 2,607,710, 3,247,017, 3,650,828, 4,152,484, 3,049,442, 3,144,352, 3,149,996, 3,242,005, 3,262,813, 3,437,510, 3,472,798, 3,808,136, 3,865,741, 3,911,196, 3,926,826, 4,000,336, 4,049,871, 4,058,646, and 4,154,895. However those which completely satisfy the above requirement have not been obtained.
For example, a mixture of a copolymer of vinyl chloride/vinyl acetate/other vinyl compound and a urethane rubber type resin is mainly used as a binder in view of dispersibility of a ferromagnetic powder, and running properties and durability of a magnetic layer. Useful copolymers of vinyl chloride/vinyl acetate/other vinyl compound include a copolymer of vinyl chloride/vinyl acetate/vinyl alcohol, a copolymer of vinyl chloride/vinyl acetate/maleic anhydride and the like. Particularly a copolymer of vinyl chloride/vinyl acetate/vinyl alcohol is often used, since the copolymer has excellent dispersibility with respect to ferromagnetic powders.
A conventional copolymer of vinyl chloride/vinyl acetate/vinyl alcohol is composed of 91 to 95 wt% of vinyl chloride, 3 to 6 wt% of vinyl acetate and 2 to 3 wt% of vinyl alcohol. This copolymer is commercially available under the trade name "VAGH" manufactured by Union Carbide Co., Ltd., "S-LEC-A" manufactured by Sekisui Kagaku Kogyo Co., Ltd., and "Denka vinyl 1000G" manufactured by Denki Kagaku Kogyo Co., Ltd. However, where this type of copolymer of vinyl chloride/vinyl acetate/vinyl alcohol is used, flexibility of a magnetic layer is short. Therefore, a large amount of a plasticizer needs to be added or a large amount of a soft resin needs to be added. However, where a large amount of a plasticizer is used, blooming readily occurs, or durability of a magnetic layer is decreased. Where a large amount of polyurethane resin is used in combination, dispersibility of ferromagnetic powders is degraded and running properties are also degraded at high temperature and high humidity.
Accordingly, it has been difficult to obtain a medium having sufficiently excellent running properties and durability by the use of the combination of a conventional copolymer of vinyl chloride/vinyl acetate/vinyl alcohol and a polyurethane resin.